This invention relates to a brake actuator incorporating an automatic adjuster and primarily for use with a disc brake. The actuator of the invention is of the general kind in which a pair of actuator components are separable by the application of hydraulic pressure and are associated respectively with first and second adjuster members which are interengaged by means of a screw threaded connection, one of said adjuster members being rotatable and the other being non-rotatable. The rotatable adjuster member is arranged to follow brake-applying movement of one of the actuator components and moves axially relative to the other adjuster member by rotation therealong in response to excessive movement of said one actuator component, clutch means normally preventing rotation of said rotatable adjuster member until adjusting movement thereof is required.
An actuator of this general kind is described in British Pat. No. 1553874 which, in FIG. 4, illustrates an actutor wherein the rotatable adjuster member is normally urged into clutching engagement with the actuator piston and in order to rotate this adjuster member manually to retract the piston for servicing purposes for example, it is necessary to rotate the piston itself. This must not only be effected against the substantial friction of the pressure seal for the piston and the piston boot, but also leads to undesirable twisting of the boot. Although the boot may be removed to obviate part of the problem this is regarded as undesirable, since ingress of dirt may result. It is also possible, with this prior arrangement, to wind back the rotatable adjuster member to such an extent that it locks against an internal rotationally fixed component of the actuator, thereby rendering the adjuster inoperable, when the brake is next applied.
An object of the present invention is to provide a brake actuator in which manual de-adjustment may be readily effected without giving rise to the aforesaid problems.